The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. The disclosure of any references cited herein is incorporated by reference.
Amperometric electrochemical gas sensors are electrochemical cells similar in structure and operation to batteries and fuel cells. As such, these three devices have several structures in common, including, (i) an anode or anode compartment (wherein electrochemical oxidation occurs), (ii) a cathode, or cathode compartment (wherein electrochemical reduction occurs), (iii) an ionically conductive electrolyte (which maintains ionic electrical contact between the two electrodes, (iv) a housing (which encloses the electrodes and electrolyte), (v) contacts or poles (which are generally metallic electrical contacts between the electrodes and external circuitry) and (vi) external electronic circuitry used in connection with these devices. Batteries and fuel cells function primarily as power sources and place few design restrictions on the circuitry to which they may be placed in electrical contact. Amperometric gas sensors often require the use of specific driving circuitry (for example, a potentiostat) for proper function. There are, however, amperometric gas sensors that function analogously to fuel cells, and require only a method of measuring the electrical current which flows between the anode and cathode in the presence of the target or analyte gas.
Batteries are self-contained electrochemical energy storage and conversion devices. They are arranged so that both the anode and the cathode include, or are in intimate electrical contact with, relatively large quantities of substances with different electrochemical energies. Significant and useful amounts of electrical current flow through electronic circuitry when the anode and cathode poles of a battery are connected to the electronic circuitry. The source of this current is the electrochemical conversions of the anode and cathode materials (oxidation and reduction). Batteries are self-contained, from an electrochemical point of view, in that they are fabricated with sufficient anode material and cathode material to provide a useful lifetime or amount of electrical energy. As such, batteries are usually well sealed. In many designs, batteries are hermetically sealed. Common examples of batteries include the Leclanche cell (the “dry” cell) and the Plante cell (the lead acid battery).
Fuel cells, on the other hand, are electrochemical energy conversion devices that require an external supply of anode material, of cathode material, or of both anode material and cathode material. The electrodes of fuel cells are usually electrocatalytic in nature. In that regard, the fuel cell electrodes provide electrochemically active surfaces to support the electrochemical reactions of the cell, but do not actually chemically participate in the reactions. Unlike a battery, the useful life of which is generally over when the electrochemically active electrode materials are consumed, a fuel cell will operate continuously as long as electrochemically active fuel (anode material) and oxidizer (cathode material) are supplied to the device. An example of a common fuel cell is the Grove cell, or hydrogen-oxygen fuel cell. In the Grove cell, hydrogen is the fuel and oxygen is the oxidizer.
Amperometric electrochemical gas sensors are special cases of fuel cells. They are typically miniature in size (compared to fuel cells used for power generation) and are designed to use a target gas of analytical interest (that is, the analyte gas) as fuel. In the absence of the target gas, there are no bulk electrochemical conversions (Faradaic reactions) occurring at the electrodes, and thus, essentially zero current flows in the sensor. When present, the analyte gas undergoes electrochemical oxidation or reduction, resulting in the generation of Faradaic currents. The resultant current flow is sensed by the external driving circuitry and provides the analytical signal of the sensor. Typically, the observed current is directly proportional to the concentration of the analyte gas.
As discussed above, batteries, fuel cells and amperometric electrochemical gas sensors are similar in may respects. However, the manufacture of amperometric gas sensors poses several unique difficulties. First, unlike batteries, there must be a gas inlet to allow the analyte gas to enter the cell. There must also be contacts or poles which carry the current from the surfaces of the electrodes to external circuitry. Finally, the sensor must be fabricated in such a way as to retain the ionic electrolyte, which is often a highly corrosive aqueous acid or base. Sealing amperometric electrochemical gas sensors against leakage of the internal liquid electrolyte, while allowing entry of the analyte gas and collection of the resultant currents, are clearly important features of the mechanical design of electrochemical gas sensors.
Oxygen sensors are a special case of amperometric electrochemical gas sensors. Typically, electrochemical oxygen sensors include a noble metal working electrode and a sacrificial metal anode, which is typically lead or zinc. Sensors of this type have been used for many years to detect and measure oxygen concentrations in a variety of applications. Lead-based sensors suffer from several disadvantages, including a limited lifetime and the use of toxic metals.
Nonetheless, the sacrificial metal anode of oxygen sensors is typically a lead anode. Oxygen that enters the sensor is reduced at the working electrode, while the lead anode is oxidized to lead oxide. The sensor operates as long as there is electrochemically accessible lead in the sensor. To increase service lifetime, the lead content must be increased or the influx of oxygen must be decreased. Each of these paths to increasing sensor lifetime has associated advantages and disadvantages. In any event, however, the lifetime of the sensor is limited by the amount of lead present therein, which is determined at the time of manufacture.
Recently, a new type of oxygen sensor, typically referred to as “oxygen pump” sensors, have been disclosed. Oxygen pump sensors do not include a sacrificial base metal anode. Instead, oxygen pump sensors include an electrocatalytic anode or counter electrode. Oxygen entering the sensor is reduced to an oxide ion at the working electrode. Concurrently, the electrolyte is oxidized at the counter electrode, producing oxygen on a one-to-one molecular basis. Oxygen sensors of this type may have much longer useful service lifetimes than those that include a sacrificial anode. However, oxygen that is produced at the counter electrode needs to be removed to ensure the proper operation of an oxygen pump sensor. If oxygen is not removed in an efficient manner, internally produced oxygen can pressurize the sensor and find its way to the working electrode, thus affecting the analytical signal of the sensor. Additionally, an increase in internal pressure may cause the liquid electrolyte to leak from the internal portions of the sensor housing.
A variety of sensors that operate on the oxygen pump principle have been developed. Those sensors include a thin porous, hydrophobic membrane to create a vent system to vent generated oxygen. While these membrane-based vent systems can create diffusion paths to vent oxygen from the interior of the sensor housing, the sensor housing must include a passage or hole that is covered by the thin porous, hydrophobic membrane. Such holes or passages are associated with an increased risk of electrolyte leakage. Moreover, the efficient functioning or operation of a membrane-based vent system can be affected by the orientation of the sensor. For example, in certain orientations, the interior surface of the membrane may be completely wetted or contacted by liquid electrolyte, which can significantly adversely affect the operation of the membrane to vent gas. Providing more than one passage/membrane vent at different positions can reduce position- or orientation-dependent effects, but can increase the potential for leakage of liquid electrolyte from the sensor.